Electronic oven



April 15, 1969 ELECTRONIC OVEN Sheet Filed June 29, 1967 7 mm \m 3 Y omwn A 0 0 "0... m w .0 0 k x m 0 W mm 6 w o m Q l v.0 Q NE E S m W6 i wKm. a m g Q a 3 mm Qwi w 5 m9 o: www v I I 11|IrlI|i J, m: n9 wwm \vn\wn HI? ///4- w: I n: E m 9 A won mmm \w 5 NR mom wmw 8L \ww v9 Q9 i 4Now N9 N9 \9 mww QQMQH m Em cow NM. Qmm m U Dvm 4. m w Y ow Q2 w 2m 8mApril 15, 1969 ELECTRON IC OVEN Sheet 3 of 5 Filed June 29, 1967 mon QmmNmm Gm mm an R? United States Patent 3,439,144 ELECTRONIC OVEN James M.Boswell, Jr., Louis H. Fitzmayer, and Richard V. Prucha, Louisville,Ky., assignors to General Electric Company, a corporation of New YorkFiled June 29, 1967, Ser. No. 649,993 Int. Cl. H05b 9/06 US. 'Cl.219-10.55 18 Claims ABSTRACT OF THE DISCLOSURE The present invention isdirected to an electronic oven, and particularly to an improvedtransmission line and filter therefor.

The present invention is particularly useful when the generator for theelectronic oven includes a magnetron deriving its operating potentialsfrom a voltage doubler and rectifier power supply wherein neither theanode nor the cathode of the magnetron may be grounded for safetypurposes. In such cases it is necessary to provide a DC. break or chokejoint in at least the outer conductor, and preferably also the innerconductor, of the transmission line coupling the output of the magnetronto the cooking cavity so that the outer conductor and the liner definingthe cooking cavity can be grounded without interfering with theoperation of the magnetron and its power supply. In such systemsditficulty has been encountered in preventing escape of the thirdharmonic and multiples of the third harmonic from the choke joint usedin providing the DC. insulation between the generator and the cookingcavity, and there also has been encountered excessive propagation of thethird harmonic and multiples thereof of the operating frequency of themagnetron to the cooking cavity.

It is an important object of the present invention to provide animproved electronic oven of the type set forth including a coaxialtransmission line having therein a choke tube surrounding the outerconductor at the choke joint providing a DC. insulation between themagnetron and the cooking cavity, the choke tube being spaced from theassociated outer conductor and having one end thereof disposed adjacentto the open end of the associated outer conductor and having the otherend thereof connected to the associated outer conductor, the distancebetween the electrical connection of the choke tube to the associatedouter conductor and the end of the choke tube adjacent to the free endof the outer conductor being equivalent to one quarter wavelength of thefrequency of operation of the magnetron to provide a high impedance atthe open end of the associated outer conductor to the third and higherharmonics of the frequency of operation of the magnetron.

Another object of the invention is to provide an electronic oven havinga transmission line including therein a pair of telescopicallyoverlapping outer conductors electrically insulated from each other toprovide a choke joint, the inner one of the overlapping outer conductorsice having a plurality of spaced-apart lands on the surface thereofdisposed toward the other outer conductor, the width of the lands andthe spaces therebetween being such that in cooperation with the otherouter conductor there is provided a choke filter for the third andhigher harmonics of the frequency of operation of the magnetron furtherto attenuate radiation of the third and higher harmonics of thefrequency of operation of the magnetron from the joint between theoverlapping outer conductors.

A further object of the invention is to provide an improved electronicoven of the type set forth having a transmission line including thereina first outer conductor and a first inner conductor coaxially arrangedand coupled to the output terminals of the magnetron, a second outerconductor electrically connected to the first outer conductor and havingthe longitudinal axis thereof disposed at an angle with respect theretoand a second inner conductor disposed within the second outer conductorand insulated therefrom and having one end thereof connected to thefirst inner conductor and having the longitudinal axis thereof disposedat an angle with respect to the first inner conductor, and a tuning slugextending longitudinally outwardly from the first inner conductor at thejuncture thereof with a second inner conductor, the tuning slugproviding a capacitance that cooperates with the distributed capacitanceand inductance in the transmission line to provide a low pass filter forfrequencies below the third harmonic and multiples of the third harmonicof the frequency of operation of the magnetron, whereby in thetransmission of electromagnetic wavelength between the first and secondinner conductors there is effected high attenuation of the thirdharmonic and multiples of the third harmonic of the frequency ofoperation of the magnetron.

Further features of the invention pertain to the particular arrangementof the parts and elements of the electronic oven and the transmissionline forming a part thereof, whereby the above-outlined and additionaloperating features thereof are attained.

The invention both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood 'by reference to the following specification taken inconnection with the accompanying drawings, in which:

FIGURE 1 is a side view with certain parts broken away of an electronicoven embodying therein the present invention;

FIG. 2 is a fragmentary view of an enlarged scale in vertical sectionwith certain portions broken away of the lower portion of thetransmission line of the present invention; and

FIG. 3 is a fragmentary view on an enlarged scale in vertical sectionwith certain portions broken away of the upper portion of thetransmission line of the present invention.

Referring to FIG. 1 of the drawings, the electronic oven 10 thereillustrated, and embodying the features of the present invention, is inthe form of a combination electric and electronic range that isespecially designed for home use. More particularly, the electronic oven10' comprises an upstanding box-like casing 11 formed of steel includinga pair of side walls 12, a rear wall 13 which is preferably removable, atop wall 14 and a bottom wall 16. The casing 11 houses in the uppercentral portion thereof a metal liner 20 defining a heating or cookingcavity 21 therein, the metal liner 20 preferably being formed of steel,and essentially comprising a box-like structure provided with a top wall22, a bottom wall 23, a rear wall 24 and a pair of oppose-d side walls25; whereby the liner 20 is provided with an upstanding front openinginto the cooking cavity 21 defined therein. Further, the casing 111 isprovided with a front door 28 arranged in a front opening formed thereinand cooperating with the front opening provided in the liner 20, thefront door 28 being mounted adjacent to the lower end thereof uponassociated hinge structure 29; whereby the front door 28 is movablebetween a substantially vertical closed position and a substantiallyhorizontal open position with respect to the front opening provided inthe liner 20.

An electric heating unit 36 is arranged in the upper portion of thecooking cavity 21 adjacent to and below the top wall 22 and an electricheating unit 37 is arranged in the lower portion of the cooking cavity21 adjacent to and above the bottom wall 23; which electric heatingunits 36 and 37 are of the sheathed resistance type and are utilized incarrying out the conventional baking and broiling cooking operations inthe cooking cavity 21. The side walls 25 of the liner 20 further carrythereon a plurality of shelf supports 26 for supporting shelfs (notshown) that in turn support items to be cooked within the cooking cavity21.

Disposed below the bottom wall 23 is a bottom machinery compartment inthe lower portion of the casing 111, a lower front panel 19 beingprovided that is removable to give access to the bottom machinerycompartment 35. Mounted in the bottom machinery compartment 35 is anelectric motor 31 having an output drive shaft 32 that drives a gearreducer 33, the output shaft of which extends upwardly through thebottom wall 23 and carries on the upper end thereof a turntable 34arranged in the lower portion of the cooking cavity 21. The turntable 34is mounted for rotation upon operation of the motor 31 and is adapted tosupport food to be heated or cooked in an electronic cooking operationthat is carried out in the cooking cavity 21. The motor 31 is operativeto turn the turntable 34 at a rate of approximately six revolutions perminute.

A rear baffle is provided to the rear of the liner 20 and extends acrossthe casing 11 between the side walls 12 thereof, the bafiie 40 beingformed of metal, such as steel, and cooperating with the casing 11 toprovide a rear machinery compartment 45, the rear machinery compartmentbeing disposed behind the liner 20 and access thereto being providedthrough the removable rear wall 13.

There is arranged in the bottom machinery compartment 35 amagnetron-type generator for supplying ultra-high frequencyelectromagnetic wave energy for electronic cooking within the cookingcavity 21, the magnetron 50 being of the construction and arrangementdisclosed in United States Letters Patent No. 3,315,121 granted Apr. 18,1967 to James E. Staats for Crossed- Field Electric Discharge Device.Referring to FIGS. 1 and 2, it will be seen that the magnetron 50 isdisposed within a box-like structure or casing 51 that extendscompletely about the magnetron 50 but is open on two opposed sidesthereof, i.e., the sides disposed to the left and right in FIGS. 1 and2. The magnetron 50 and the associated parts therefor are electricallyinsulated from the casing 51 since high operating D.C. potentials areprescut on those parts electrically connected directly to the magnetron50. Preferably the openings of the casing 51 are covered withreticulated metal members or screens 52 that permit passage of coolingair therethrough and about the magnetron 50 but prevent contact of theuser with the magnetron 50 and the parts disposed thereabout that carrythe high operating D.C. potentials.

The magnetron 50 includes an outer wall 55 that is directly connected tothe anode thereof and which is provided at the upper end thereof with anend bell 56 carrying thereon an inner output terminal 57, the exteriorsurface of the end bell 56 providing an annular outer output terminalfor the magnetron 50. Surrounding the end bell 56 is an annular magneticpole piece 58 that is connected to a yoke 59 providing magnetic couplingto a second pole piece disposed at the other end of the magnetron 50 toestablish a magnetic field through the magnetron 50, a magnetic fieldcoil 60 surrounding the magnetic pole piece 58 to create a strongmagnetic field therethrough. As illustrated, an O-ring 61 of siliconerubber is disposed between the field coil 60 and the end bell 56. Anelectrical connection is made to the annular outer output terminalprovided by the end bell 56 by means of a wire mesh ring 62 resting onthe upper surfaces thereof and in turn resting against a flange 63formed on the lower end of an annular outer conductor 64, a secondannular member 65 having an outwardly flared flange 66 being providedaround the annular conductor 64 and resting upon the flange 63 thereofand having a wire mesh ring 67 disposed between the outer conductor 6-4and the flange 66. The inner output terminal 57 has threadedly connectedthereto an upstanding conductor 68 that is disposed within the annularouter conductor 64 and generally concentric therewith and extendingupwardly through an opening in a ceramic insulator 69 that rests uponthe upper end of the outer conductor 64. It will be understood that theRF output from the magnetron 50 appears between the outer conductor 64and the inner conductor 68 serving as the components of a coaxialtransmission line for ultra-high frequency electromagnetic wave energy.

A transmission line is provided to interconnect the output of themagnetron 50 to the cooking cavity 21, the transmission line beingparticularly illustrated in FIGS. 2 and 3 of the drawings and includinga short vertical section 100, a first horizontal section 200, a secondlonger vertical section 300 and an upper horizontal section 400connecting to an antenna extending into the rear of the cooking cavity21 for the introduction of microwave energy thereinto. Morespecifically, it will be seen that the horizontal transmission linesection 200 extends essentially rearwardly from the magnetron 50 andthrough the wall 40 and into the rear machinery compaltment 45 and thereconnects with the upstanding transmission line section 300 which in turnconnects with the horizontal transmission line 400 that extendsforwardly through the wall 40 and to the rear wall 24 of the cookingcavity 21 for connection to the antenna 80 disposed therein.

Referring specifically to FIG. 2 of the drawings, it will be seen thatthe transmission line section comprises a T 101 including a body 102having a pair of arms 104 and 10 6 and a leg or arm 108, each providedwith a cooperating internal seat or shoulder 105, 107 and 109,respectively. More particularly, the leg 108 is positioned downwardlyand receives telescopically therein the upper end of the annular outerconductor 64, the ceramic spacer 69 also being disposed therein andabutting against the shoulder 109, the lower end of the leg 108 pressingagainst the wire mesh gasket 67 whereby the outer conductor 64 iscoupled to the leg 10-8. Disposed within the T 101 is a bullet 110having a lower end 111 formed as a plurality of prongs that fit into theannular opening in the upper end of the inner conductor 68, a screw 112extending into an opening in the upper end of the bullet 110 anddownwardly therethrough and forcing the prongs on the lower end 111outwardly firmly to connect to the conductor 68, thereby to provide anelectrical connection therewith; preferably a lock washer 113 isprovided to hold the screw 112 in the adjusted position thereof. it willbe understood that a suitable opening is formed in the T 101 above thescrew 112 to provide access thereto during the assembly operation and aspring 114 is also provided in the conductor 68, the spring 114 beinguseful during assembly of the parts to assure the proper positioning ofthe bullet 110 with respect to the conductor 68. Finally, a dust cap 115is provided closing the outer end of the leg 104.

The horizontal transmission line section 200 includes a first tube orouter conductor 201, a first inner conductor 220, a second tube or outerconductor 240 and a second inner conductor 250 which is also preferablyin the form of a tube. The outer conductor 201 is annular in form andincludes a mounting portion 202 having an external diameter slightlyless than the internal diameter of the leg 106 of the T 101 andtelescopically received therein and abutting against a washer 203 ofpolytetrafiuoroethylene resin, the preferred resin being that sold underthe trademark Teflon positioned against the shoulder 107 in the leg 106.A suitable clamp 204 is provided fixedly to interconnect the leg 106 andthe mounting portion 202, thus firmly to mount the outer conductor 201on the T 101 and to provide a good electrical connection therebetween.The portion of the outer conductor 201 extending beyond the leg 106 andto the right in FIG. 2 has a first recess therein as at 205 and a secondrecess therein as at 207, the recesses 205 and 207 cooperating with theouter free end of the conductor 201 to provide a pair of spaced-apartlands 206 and 208.

The inner conductor 220 includes a body portion 221 disposed within theT 101 and having integral therewith an ear 222 having an openingtherethrough receiving the shank ofthe screw 112, the washer 113clamping the car 222 against the bullet 110 to provide a good mechanicalconnection and a good electrical connection between the bullet 110 andthe inner conductor 220. Extending outwardly to the right from the body221 and integral therewith is a cylindrical probe 225 that extendsthrough an opening in the washer 203 and generally concentric with andspaced from the inner surface 209 of the outer conductor 201. A firstring 226 of low-loss dielectric material is carried by the probe 225adjacent to the insulator 203 and in surrounding relationship therewith,and a second ring 227 of low-loss dielectric material is carried by theprobe 225 intermediate the ends thereof but disposed toward the outerend thereof and in surrounding relationship therewith. The rings 226 and227 are firmly affixed to the probe 225, the preferred material ofconstruction of the rings 226 and 227 being a synthetic organic plasticresin, the preferred resin being polytetrafluoroethylene resin.

The outer conductor 201 and the inner conductor 220 form a section of acoaxial transmission line to which are applied both the RF output fromthe magnetron 50 and the D.C. operating potentials applied thereto. Inorder to isolate electrically with respect to D.C. potentials themagnetron 50 from the cooking cavity 21, and more particularly to makeit possible to ground the outer conductor of the transmission line andthe liner 20 defining the cooking cavity 21, it is necessaryelectrically to insulate with respect to D.C. potentials the outerconductor 201 from the cooperating outer conductor 240 and alsopreferably electrically to insulate with respect to DC. potentials theinner conductor 220 from the cooperating inner conductor 250. To thisend a layer or sleeve 210 of electrically insulating and dielectricmaterial is provided covering the major portion of the length of theconductor 201, and specifically that portion thereof which istelescopically received within the cooperating outer conductor 240. Morespecifically, the inner diameter of the outer conductor 240 is slightlygreater than the outer diameter of the insulating sleeve 210 so as toreceive the associated end of the outer conductor 201 and thesurrounding sleeve 210 therein in telescoping relationship. Also, theouter or lefthand end of the outer conductor 240 is outwardly flared asat 241 so as to provide for easy assembly of the outer conductor 240over the inner conductor 201 and its enclosing insulating sleeve 210.

The inner conductor 250 is tubular in form and has an inner diameteronly slightly greater than the outer diameter of the rings 226 and 227carried by the inner conductor 228, the lefthand end of the innerconductor 250 telescopically receiving therein the adjacent end of theinner conductor 220 and more specifically the probe 225 and the rings226 and 227 carried thereon, the lefthand end of the conductor 250abutting against the insulator 203.

Finally, there is provided about the outer conductor 240 a choke sleeveor tube 270 having an inner diameter substantially greater than theouter diameter of the outer conductor 240 and spaced uniformly therefromtherearound a predetermined distance and having the lefthand end thereofas viewed in FIG. 2 terminated a short distance away from the outwardlyflared portion 241, all for reasons that will be described in detailhereinafter.

The vertically extending transmission line section 300 includes at thelower end thereof a T 301 including a body 302 having a pair of arms 304and 306 and a leg or arm 308 each provided with a cooperating internalseat or shoulder 305, 307 and 309, respectively. More specifically, theleg 304 is disposed to the left as viewed in FIG. 2 and carries thereina washer or insulator 311 of Teflon disposed against the shoulder 305and extends from the rear machinery compartment 45 through an opening 41in the wall 40 and into the bottom machinery compartment 35. The innerdiameter of the leg 304 is slightly greater than the outer diameter ofthe adjacent end of the outer conductor 240 so that the adjacent orrighthand end of the outer conductor 240 is telescopically received inthe leg 304 and abuts against the ceramic washer 311. The outer diameterof the leg 304 is slightly smaller than the inner diameter of the choketube 270 so that the choke tube 270 is telescopically receivedthereover. An annular clamp 312 is provided about the adjacent end ofthe choke tube 270 to clamp the choke tube 270 on the leg 304 and toclamp the leg 304 against the adjacent end of the outer conductor 240 toprovide a good mechanical connection between the outer conductor 240,the choke tube 270 and the T 301, and also to provide for goodelectrical connection between the outer conductor 240 and the choke tube270 at the outer end of the leg 304-. The righthand end of the innerconductor 250 as viewed in FIG. 2 extends outwardly and to the rightthrough the opening disposed centrally in the washer 311, the free endof the inner conductor 250 fitting over and afiixed to a shoulderedmember 321. A tuning slug 320 carries a screw 322 therein for thepurpose of aflixing the tuning slug 320 to the member 321 and theconductor 250.

The vertical transmission line section 300 further includes an outerannular conductor 330 and an inner annular conductor 340 extendingupwardly from the T 301 to a second T 351 (see FIG. 3). Morespecifically, the lower end of the outer conductor has an outer diameterthat is slightly less than the inner diameter of the leg 308 and istelescopically received therein and bears downwardly against a washer orinsulator 331 of Teflon supported on the shoulder 309 of the T 301. Anannular clamp 332 is provided about the leg 308 to clamp it about thelower end of the outer conductor 330 to provide both a good mechanicalconnection and a good electrical connection therebetween. The lower endof the inner conductor 340 extends downwardly through an openingdisposed centrally in the ceramic washer 331 and carries a connectingportion 341 that is connected to the tuning slug 320 so that the innerconductor 340 and the tuning slug 320 are connected both mechanicallyand electrically. There also is provided a dust cap 342 to close the leg306.

The upper end of the vertical transmission line section 300 is providedby the T 351 that includes a body 352 having a pair of arms 354 and 356and a leg or arm 358 each provided with a cooperating internal seat orshoulder 355, 357 and 359, respectively. More particularly, the leg 358is positioned downwardly and has an internal diameter slightly greaterthan the external diameter of the upper end of the outer conductor 330,whereby the upper end of the outer conductor 330 is telescopicallyreceived in the leg 358, a ceramic washer or insulator 361 beingprovided abutting against the shoulder 359 and against the upper end ofthe outer conductor 330. In order to provide both a good mechanicalconnection and a good electrical connection between the leg 358 and theouter conductor 330, an annular clamp 362 is provided about the lowerend of the leg 358 to clamp it tightly about the upper end of the outerconductor 330. The upper end of the inner conductor 340 extends upwardlythrough an opening disposed centrally in the insulating washer 361 andinto the body 352 of the T 351, a connecting portion 342 being providedon the upper end thereof. The outer end of the leg 356 is closed by adust cover 363.

The upper horizontal transmission line section 400 comprises a tube orouter conductor 401 and an inner conductor 410. The outer diameter ofthe outer conductor 401 is slightly less than the inner diameter of theleg 354 on the T 351 and is telescopically received therein and abutsagainst a ceramic washer or insulator 402 that bears against theshoulder 355 in the leg 354. It will be noted that the leg 354 extendsforwardly through an opening 42 in the wall 40 and thus from the rearmachinery compartment 45 to the rear wall 24 of the liner 20 and throughan opening therein. The forward or lefthand end of the outer conductor401 is viewed in FIG. 3 has an outwardly directed flange 403 therearoundthat is suitably secured to the rear wall 24 of the liner 20. Therighthand end of the outer conductor 401 is held in the operativeposition with respect to the leg 354 by an annular clamp 409 disposedabout the forward end of the leg 354 and clamping it against theassociated end of the outer conductor 401. Covering the outer end of theouter conductor 401 is a ceramic washer or insulator 404 that is held inplace by a mounting member 405 that includes a flange 406 secured to theflange 403, an annular body 407 extending to the left and an inwardlydirected retaining flange 408 that serves to hold the insulator 404against the flange 403 and centered With respect to the outer conductor401.

The inner conductor 410 is in the form of. a rod that extends throughaligned openings in the insulators 402 and 404, the rear end of theinner conductor 410 being secured as by the screw 411 to a connectingportion 342 at the upper end of the inner conductor 340. The forward endof the rod 410 has a threaded opening 412 therein that receives athreaded stud 81 on the rear end of the antenna 80, the antenna 80 beingin the form of a probe extending into the cooking cavity 21 defined bythe liner 20.

A coaxial transformer is provided by the parts in the transmission linesection 200 to transform the effective impedance of the cooking cavity21, the transmission line v and the magnetron 50 to match the higherimpedance of the average load in the cooking cavity 21. Morespecifically, the transformer determines the coupling between themagnetron 50 and the cooking cavity 21 and serves to decouple the loadin the cooking cavity 21 from the magnetron 50 to improve the stabilityof operation of the electronic oven. The critical parts in the coaxialtransformer are the outer conductor 201 and the inner conductor 250, theratio between the inner diameter of the outer conductor 201 and theouter diameter of the inner conductor 250 being approximately 1.5 :1,thereby providing the necessary transformer action and decoupling asdescribed.

As has been explained heretofore, the preferred power supply for themagnetron 50 is a voltage doubler and rectifier system which can beconnected directly to a household three-wire Edison supply to providethe necessary potentials for operating the magnetron 50 without the useof any power transformer therein. A voltage doubler system, however,cannot have either of the output terminals thereof grounded, wherebyboth the output terminal in the form of the conductor 64 and the outputconductor 68 from the magnetron 50 carry thereon high potentials withrespect to ground potential during the operation of the magnetron 50.For safety reasons it is essential that the oven liner 20 and thevarious parts connected thereto be grounded, and therefore it isnecessary to provide a DC. decoupling insulation in the transmissionline between the liner 20 and the magnetron 50 so as to prevent theapplication of the high potentials on the magnetron 50 to the oven liner20. Accordingly, a DC. insulation break has been provided in the outerconductor, and specifically, the insulating sleeve 210 has been providedbetween the outer conductor 201 and the outer conductor 240, wherebythere is a transmission of microwave energy therebetween but notransmission of D0. potentials therebetween. Likewise, the innerconductor 220 is insulated with respect to the inner conductor 250 so asto prevent transmission of DC. potential therebetween while providingfor an efficient transmission of microwave energy therebetween. It willbe understood therefore that the outer conductor 240 and the innerconductor 250 are insulated wtih respect to D.C. potentials from theassociated outer conductor 201 and inner conductor 220, respectively,while propagating therebetween the RF energy from the magnetron 50 tothe oven cavity 21.

These overlapping conductors 201-240 and 220-250 also provide a filterfor the second and fourth harmonic of the fundamental operatingfrequency of the magnetron 50, substantially to attenuate thetransmission of the second and fourth harmonics from the magnetron 50 tothe cooking cavity 21. To this end the outer conductors 201 and 240overlap a length that is substantially equivalent to a quarterwavelength of the operating frequency of the magnetron 50, and the innerconductors 220 and 250 likewise overlap a length equivalentsubstantially to one quarter wavelength of the operating frequency ofthe magnetron 50. The provision of the ring 227 on the probe 225 alsocauses preferential attenuation of the fourth harmonic of thefundamental operating frequency of the magnetron 50, the ring 227 beingpositioned a predetermined distance from the outer end of the probe 225equal substantially to a quarter wavelength of the fourth harmonic ofthe fundamental operating frequency of the magnetron 50. As a result,the overlapping relationship of the outer conductors 201 and 240, theoverlapping relationship of the inner conductors 220 and 250 and theposition of the rings 226 and 227 on the probe 225 serve to provide anefficient filter for the second and fourth harmonics of the fundamentaloperating frequency of the magnetron 50, thus highly to attenuate thetransmission of the second and fourth harmonics from the magnetron 50 tothe cooking cavity 21.

It will be appreciated that the construction of the horiozntaltransmission line section 200 is such as to permit ready assembly anddisassembly between the parts thereof. More specifically, the outerconductor 240 is slidingly received over the insulating sleeve 210 andcan be readily removed therefrom by movement to the right as viewed inFIG. 2. Likewise, the inner conductor 250 is simply slidingly fittedover the probe 225 and the rings 226 and 227 thereon, whereby the innerconductor 250 can be removed therefrom by simply moving it to the rightas viewed in FIG. 2. In short, those parts mounted on and connected tothe T 301 can be bodily moved to the right for dissassem'bly withrespect to those parts mounted on the T 101, thus providing for a readydisassembly of the transmission line section 200 into two telescopingparts. To facilitate assembly of the two telescoping transmission linesections, the forward or righthand end of the ring 227 is beveled as at228 to facilitate the insertion of the conductor 250 thereover, and theouter conductor 240 has the lefthand end thereof flared outwardly as at241 to facilitate assembly thereof over the insulating sleeve 210. Inorder to hold the parts in the assembled position with the lefthand endof the inner conductor 250 firmly pressed against the insulating washer203, a spring 370 has been provided interconnecting the T 301 and thewall 40, the spring 370 being under tension and urging the T 301 and theparts mounted thereon to the left as viewed in-FIG. 2 and into theassembled position thereof.

While the above described choke joint structure admirably fulfills thedesired purpose of providing a DC. insulation between the outerconductors 201-240 and between the inner conductors 220-250, whileproviding for ready assembly and disassembly thereof, a problem isencountered in that there is a tendency for the third and higherharmonics of the fundamental operating frequency of the magnetron 50' toescape from the resultant choke joint to an undesirable degree.Accordingly, there has been provided in accordance with the presentinvention the coke tube 270 and a choke filter formed by the outerconductors 201 and 240 substantially to attenuate and essentially toeliminate the propagation of the third harmonic and higher harmonics ofthe operating frequency of the magnetron 50 from the choke joint. Referring first to the choke tube 270, the righthand end thereof as viewed inFIG. 2 is electrically connected to or shorted to the outer conductor240 at the lefthand end of the leg 304 on the T 301, and the distancebetween this point of shorting and the forward or lefthand end of thechoke tube 270 is equal to one quarter wavelength of the fundamentaloperating frequency of the magnetron 50. The dimension recited offers ahigh impedance to the third and higher harmonics at the lefthand end ofthe choke tube 270 in the space between the choke tube 270 and the outerconductor 240 and particularly the flared end 241 thereof. The choketube 270 also isolates the outer conductor 240 from the field coilhousing or magnetic yoke 59, thus preventing any transportation of thirdharmonic wave energy in the space between the choke tube 270 and theyoke 59. As a result there is substantially no leakage of the third andhigher harmonics from the juncture between the outer conductors 201 and240, although there is a low impedance for the fundamental frequency ofthe magnetron 50 at the outer end of the inner conductor 201, thereby toprovide for efficient transmission of the fundamental frequency from theouter conductor 201 to the outer conductor 240 for further transmissionto the cooking cavity 21.

The choke filter provided .by the outer conductor 201 in cooperationwith the outer conductor 240 provides a short for the third and higherharmonics of the fundamental operating frequency of the magnetron 50- soas further to attenuate the radiation of these frequencies from thechoke joint between the outer conductors 201 and 240, and specificallyat the flared end 241 of the outer conductor 240. This choke filter isprovided by the lands 206 and 208 and the spaces 205 and 207, thedimensions of these parts being adjusted and arranged so that thedistances between any two points of impedance discontinuity on the lands206 and 208 are integral multiples of one-half the wavelength of thethird, fourth, fifth, sixth, seventh and eight harmonics of thefundamental operating frequency of the magnetron 50. More specially, thedimensions of the outer conductor 201 are chosen and arranged so thatthe width of the lands 206 and 208 is equal to approximately 0.039 timesthe fundamental wavelength of the magnetron 50, the space 207 has awidth equal to 0.049 times the fundamental wavelength of the magnetron50, the overall length of the outer conductor 201 is equal to onequarter wavelength of the fundamental frequency of the magnetron 50 andthe internal diameter of the outer conductor 201 is equal to 0.058 timesthe fundamental wavelength of the magnetron 50. When the parts are sodimensioned, there can be no points of resonance for the wavelength orone-half the wavelength of the third, fourth, fifth, sixth, seventh andeighth harmonics of the fundamental frequency of the magnetron 50 in thechoke filter. Accordingly, there is a high attenuation of the third andhigher harmonics of the fundamental frequency of the magnetron 50 inpassing from the outer conductor 201 to the outer conductor 240, thushighly to attenuate radiation of the third and higher harmonics of thefundamental frequency of the magnetron 50 from the joint therebetweenand specifically at the flared end 241 of the outer conductor 240.

Turning now to the third harmonic filter provided by the tuning slug 320in FIG. 2, the tuning slug 320 adds capacitance to the transmission lineat the right angle turn or juncture of the inner conductors 250 and 340in the T 301. This added capacitance acts with existing distributedcapacitance and inductance in the transmission line to give a rejectionfilter or notch filter effect at the third harmonic and multiples of thethird harmonic of the fundamental frequency of the magnetron 50. Morespecifically, there is provided at the righthand end of the tuning slug320 a high impedance to the third harmonic and multiples thereof and atthe connector 341 a low impedance to the third harmonic and multiplethereof which tends to reflect the third harmonic and multiples thereofback toward the magnetron 50, thus substantially to attenuate andprevent transmission of the third harmonic and multiples thereof fromthe inner conductor 250 onto the inner conductor 340. As a consequence,the tuning slug 320 has the effect of effectively filtering the thirdharmonic and multiples thereof from the energy transmitted into thetransmission line section provided by the conductor 330-340, thus highlyto attenuate the transmission of the third harmonic and multiplesthereof of the operating frequency of the magnetron '50 to the cookingcavity 21.

In a typical construction of the electronic oven 10, the TS 101, 301 and351 are formed of copper, the outer conductor 201, the inner conductor220, the inner conductor 250 and the inner conductor 340 are formed ofaluminum. The outer conductor 240, the choke sleeve 270 and the outerconductor 330 are formed of copper. The outer conductor 401, the innerconductor 410, and the antenna are formed of nickel plated steel. In apreferred embodiment of the invention, the magnetron 50 has a frequencyof operation of 915 megacycles in which case the outer conductor 201 hasan overall length of 3.25 inches, an outer diameter of the mountingportion 202 of 1.125 inches, an inner diameter of 0 .75 inch, anexternal diameter of the lands 206 and 208 of 1.028 inches, an externaldiameter of the spaces 205 and 207 of 0.970 inch, a length of the lands206 and 208 of 0 .50 inch and a length of the space 207 of 0.63 inch.

From the above it will be seen that there has been provided anelectronic oven which fulfills all of the objects and advantages setforth above.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications can be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. An electronic oven comprising a box-like metal liner defining acooking cavity, a generator for supplying ultrahigh frequencyelectromagnetic energy of a predetermined frequency, said generatorhaving an annular outer output terminal and an inner output terminalboth at high potentials with respect to ground potential, and a coaxialtransmission line interconnecting said generator and said cooking cavityand including a top section coupled to said cooking cavity and a bottomsection coupled to said generator and an intermediate sectioninterconnecting said top and bottom sections, said top section includinga first tube connected to said liner and extending outwardly therefromto provide a first outer conductor and a first inner conductor disposedwithin said first tube and insulated therefrom and having the outer endthereof coupled to said cooking cavity, said bottom section including asecond tube connected to said annular outer output terminal andproviding a second outer conductor and a second inner conductorconnected to said inner output terminal and disposed within said secondtube and insulated therefrom, a third tube telescopically overlappingsaid second tube and insulated therefrom and extending outwardlytherefrom to provide a third outer conductor, a third inner con-ductordisposed within said third tube and insulated therefrom andtelescopically overlapping said second inner conductor and insulatedtherefrom, said intermediate section including a fourth tube providing afourth outer conductor interconnecting and electrically connected toboth said first tube and said third tube and having the longitudinalaxis thereof disposed at an angle with respect to said third tube, afourth inner conductor dispose-d within said fourth tube and insulatedtherefrom and having one end thereof connected to said first innerconductor and having the other end thereof connected to said third innerconductor and having the longitudinal axis thereof disposed at an anglewith respect to said third inner conductor, the end of said third tubedisposed toward said generator being outwardly flared to facilitateinsertion thereinto of the adjacent end of said second tube, a choketube surrounding said third tube and having one end thereof disposedadjacent to the flared end of said third tube and spaced away therefromand having the other end thereof electrically connected to said thirdtube, the distance between the electrical connection of said choke tubeto said third tube and the end of said choke tube adjacent to the flaredend of said third tube being equivalent to one quarter wavelength of thefrequency of operation of said generator to provide a high impedance atthe flared end of said third tube to the third and higher harmonics ofthe frequency of operation of said generator thereby highly to attenuateradiation of the third and higher harmonies of the frequency ofoperation of said generator from the joint between said second and thirdtubes, said second tube having a plurality of spaced-apart lands on thesurface thereof disposed toward said third tube, the width of said landsand the spaces therebetween being such that in cooperation with saidthird tube there is provided a choke filter for the third and higherharmonics of the frequency of operation of said generator further toattenuate radiation of the third and higher harmonics of the frequencyof operation of said generator from the joint between said second andthird tubes, and a tuning slug extending longitudinally outwardly fromsaid third inner conductor at the juncture thereof with said fourthinner conductor, said tuning slug providing a capacitance thatcooperates with the distributed capacitance and inductance in saidtransmission line to provide a rejection filter for the third harmonicof the frequency of operation of said generator and multiples of saidthird harmonic, whereby in the transmission of the electromagnetic waveenergy between said third and fourth inner conductors there is effectedhigh attenuation of the third harmonic of the frequency of operation ofsaid generator and multiples of said third harmonic.

2. The electronic oven set forth in claim 1, wherein said second tubeand said third inner conductor cooperate to provide a coaxialtransformer to transform the impedance of said generator and saidtransmission line to match the higher impedance of the average load insaid cooking cavity, the ratio between the inner diameter of said secondtube and the outer diameter of said third inner conductor beingapproximately 1.5 :l.

3. The electronic oven set forth in claim 2, wherein the inner diameterof said second tube is equivalent in length to 0.058 times thepredetermined wavelength of operation of said generator.

4. An electronic oven comprising a box-like metal liner defining acooking cavity, a generator for supplying ultra-high frequencyelectromagnetic energy of a predeto said generator and an intermediatesection interconnecting said top and bottom sections, said top sectionincluding a first tube connected to said liner and extending outwardlytherefrom to provide a first outer conductor and a first inner conductordisposed within said first tube and insulated therefrom and having theouter end thereof coupled to said cooking cavity, said bottom sectionincluding a second tube connected to said annular outer output terminaland providing a second outer conductor and a second inner conductorconnected to said inner output terminal and disposed within said secondtube and insulated therefrom, a third tube telescopically overlappingsaid second tube and insulated therefrom and extending outwardlytherefrom to provide a third ou'ter conductor, a third inner conductordisposed within said third tube and insulated therefrom andtelescopically overlapping said second inner conductor and insulatedtherefrom, said intermediate section including a fourth tube providing afourth outer conductor interconnecting and electrically connected toboth said first tube and said third tube and having the longitudinalaxis thereof disposed at an angle with respect to said third tube, afourth inner conductor disposed within said fourth tube and insulatedtherefrom and having one end thereof connected to said first innerconductor and having the other end thereof connect-ed to said thirdinner conductor and having the longitudinal axis thereof disposed at anangle with respect to said third inner conductor, the end of said thirdtube disposed toward said generator being outwardly flared to facilitateinsertion thereinto of the adjacent end of said second tube, and a choketube surrounding said third tube and having one end thereof disposedadjacent to the flared end of said third tube and spaced away therefromand having the other end thereof electrically connected to said thirdtube, the distance between the electrical connection of said choke tubeto said third tube and the end of said choke tube adjacent to the flaredend of said third tube being equivalent to one quarter wavelength of thefrequency of operation of said generator to provide a high impedance atthe flared end of said third tube to the third and higher harmonics ofthe frequency of operation of said generator thereby highly to attenuateradiation of the third and higher harmonics of the frequency ofoperation of said generator from the joint between said second and thirdtubes, said second tube having a plurality of spaced apart lands on thesurface thereof disposed toward said third tube, the width of said landsand the spaces therebetween being such that in cooperation with saidthird tube there is provided a choke filter for the third and higherharmonics of the frequency of operation of said generator further toattenuate radiation of the third and higher harmonics of the frequencyof operation of c said generator from the joint between said second andthird tubes.

5. The electronic oven set forth in claim 4, wherein said lands and thespaces therebetween are arranged so that the distances between any tWopoints of impedance discontinuity on adjacent lands are multiples of thewavelength or one-half the wavelength of the third and fourth and fifthand sixth and seventh and eighth harmonics of the predeterminedfrequency of operation of said generator.

6. The electronic oven set forth in claim 4, wherein two of said landsare provided having a space therebetween and on each side thereof, theWidth of each of said lands being equivalent in length to about 0.039times the wavelength of the predetermined frequency of operation of saidgenerator, and the width of the space between said lands beingequivalent in length to about 0.049 times the wavelength of thepredetermined frequency of operation of said generator.

7. An electronic oven comprising a box-like metal liner defining acooking cavity, a generator for supplying ultrahigh frequencyelectromagnetic energy of a predetermined frequency, said generatorhaving an annular outer output terminal and an inner output terminalboth at high potentials with respect to ground potential, and a coaxialtransmission line interconnecting said generator and said cooking cavityand including a top section coupled to said cooking cavity and a bottomsection coupled to said generator and an intermediate sectioninterconnecting said top and bottom sections, said top section includinga first tube connected to said liner and extending outwardly therefromto provide a first outer conductor and a first I inner conductordisposed within said first tube and insulated therefrom and having theouter end thereof coupled to said cooking cavity, said bottom sectionincluding a second tube connected to said annular outer output terminaland providing a second outer conductor and a second inner conductorconnected to said inner output terminal and disposed within said secondtube and insulated therefrom, a third tube telescopically overlappingsaid second tube and insulated therefrom and extending outwardlytherefrom to provide a third outer conductor, a third inner conductordisposed within said third tube and insulated therefrom andtelescopically overlapping said second inner conductor and insulatedtherefrom, said intermediate section including a fourth tube providing afourth outer conductor interconnecting and electrically connected toboth said first tube and said third tube and having the longitudinalaxis thereof disposed at an angle with respect to said third tube, afourth inner conductor disposed within said fourth tube and insulatedtherefrom and having one end thereof connected to said first innerconductor and having the other end thereof connected to said third innerconductor and having the longitudinal axis thereof disposed at an anglewith respect to said third inner conductor, the end of said third tubedisposed toward said generator being outwardly flared to facilitateinsertion thereinto of the adjacent end of said second tube, a choketube surrounding said third tube and having one end thereof disposedadjacent to the flared end of said third tube and spaced away therefromand having the other end thereof electrically connected to said thirdtube, the distance between the electrical connection of said choke tubeto said third tube and the end of said choke tube adjacent to the flaredend of said third tube being equivalent to one quarter wavelength of thefrequency of operation of said generator to provide a high impedance atthe flared end of said third tube to the third and higher harmonics ofthe frequency of operation of said generator thereby highly to attenuateradiation of the third and higher harmonies of the frequency ofoperation of said generator from the joint between said second and thirdtubes, and a tuning slug extending longitudinally outwardly from saidthird inner conductor at the juncture thereof with said fourth innerconductor, said tuning slug providing a capacitance that cooperates withthe distributed capacitance and inductance in said transmission line toprovide a low pass filter for frequencies below the third harmonic ofthe frequency of operation of said generator and multiples of said thirdharmonic, whereby in the transmission of the electromagnetic wave energybetween said third and fourth inner conductors there is effected highattenuation of the third harmonic of the frequency of operation of saidgenerator and multiples of said third harmonic.

8. The electronic oven set forth in claim 7, wherein said second tubeand said third tube telescopically overlap a distance equivalent to aquarter wavelength of the predetermined frequency of operation of saidgenerator, and said second inner conductor and said third innerconductor telescopically overlap a distance equivalent to a quarterwavelength of the predetermined frequency of operation of saidgenerator, thereby to provide high attenuation of the second harmonicand multiples thereof of the predetermined frequency of operation ofsaid generator in transmission of electromagnetic wave energy along saidtransmission line.

9. An electronic oven comprising a box-like metal liner defining acooking cavity, a generator for supplying ultrahigh frequencyelectromagnetic energy of a predetermined frequency, said generatorhaving an annular outer output terminal and an inner output terminalboth at high potentials with respect to ground potential, and a coaxialtransmission line interconnecting said generator and said cooking cavityand including a top section coupled to said cooking cavity and a bottomsection coupled to said gene erator and an intermediate sectioninterconnecting said top and bottom sections, said top section includinga first tube connected to said liner and extending outwardly therefromto provide a first outer conductor and a first inner conductor disposedwithin said first tube and insulated therefrom and having the outer endthereof coupled to said cooking cavity, said bottom section including asecond tube connected to said annular outer output terminal andproviding a second outer conductor and a second inner conductorconnected to said inner output terminal and disposed within said secondtube and insulated therefrom, a third tube telescopically overlappingsaid second tube and insulated therefrom and extending outwardlytherefrom to provide a third outer conductor, a third inner conductordisposed within said third tube and insulated therefrom andtelescopically overlapping said second inner conductor and insulatedtherefrom, said intermediate section including a fourth tube providing afourth outer conductor interconnecting and electrically connected toboth said first tube and said third tube and having the longitudinalaxis thereof disposed at an angle with respect to said third tube, afourth inner conductor disposed Within said fourth tube and insulatedtherefrom and having one end thereof connected to said first innerconductor and having the other end thereof connected to said third innerconductor and having the longitudinal axis thereof disposed at an anglewith respect to said third inner conductor, the end of said third tubedisposed toward said generator being outwardly flared to facilitateinsertion thereinto of the adjacent end of said second tube, said secondtube having a plurality of spaced apart lands on the surface thereofdisposed toward said third tube, the width of said lands and the spacestherebetween being such that in cooperation with said third tube thereis provided a choke filter for the third and higher harmonics of thefrequency of operation of said generator further to attenuate radiationof the third and higher harmonics of the frequency of operation of saidgenerator from the joint between said second and third tubes, and atuning slug extending longitudinally outwardly from said third innerconductor at the juncture thereof with said fourth inner conductor, saidtuning slug providing a capacitance that cooperates with the distributedcapacitance and inductance in said transmission line to provide a lowpass filter for frequencies below the third harmonic of the frequency ofoperation of said generator and multiples of said third harmonic,whereby in the transmission of the electromagnetic wave energy betweensaid third and fourth inner conductors there is effected highattenuation of the third harmonic of the frequency of operation of saidgenerator and multiples of said third harmonic.

10. The electronic oven set forth in claim 9, wherein the lands and thespaces therebetween are arranged so that the distances between any twopoints of impedance discontinuity on adjacent lands are multiples of thewavelength or one-half the wavelength of the third and fourth and fifthand sixth and seventh and eighth harmonics of the predeterminedfrequency of operation of said generator.

11. The electronic oven set forth in claim 9, wherein tWo of said landsare provided having a space therebetween and on each side thereof, thewidth of each of said lands being equivalent in length to about 0.039times the wavelength of the predetermined frequency of operation of saidgenerator, and the width of the space between said lands beingequivalent in length to about 0.049 times the wavelength of thepredetermined frequency of operation of said generator.

12. The electronic oven set forth in claim 9, wherein said second tubeand said third tube telescopically overlap a distance equivalent to aquarter wavelength of the predetermined frequency of operation of saidgenerator, and said second inner conductor and said third innerconductor telescopically overlap a distance equivalent to a quarterwavelength of the predetermined frequency of operation of saidgenerator, thereby to provide high attenuation of the second harmonicand multiples thereof of the predetermined frequency of operation ofsaid generator in the transmission of electromagnetic wave energy alongsaid transmission line.

13. An electronic oven comprising a box-like metal liner defining acooking cavity, a generator for supplying ultra-high frequencyelectromagnetic energy of a predetermined frequency, said generatorhaving an annular outer output terminal and an inner output terminalboth at high potentials with respect to ground potential, and a coaxialtransmission line interconnecting said generator and said cooking cavityand including a top section coupled to said cooking cavity and a bottomsection coupled to said generator and an intermediate sectioninterconnecting said top and bottom sections, said top section includinga first tube connected to said liner and extending outwardly therefromto provide a first outer conductor and a first inner conductor disposedwithin said first tube and insulated therefrom and having the outer endthereof coupled to said cooking cavity, said bottom section including asecond tube connected to said annular outer output terminal andproviding a second outer conductor and a second inner conductorconnected to said inner output terminal and disposed within said secondtube and insulated therefrom, a third tube telescopically overlappingsaid second tube and insulated therefrom and extending outwardlytherefrom to provide a third outer conductor, a third inner conductordisposed within said third tube and insulated therefrom andtelescopically overlapping said second inner conductor and insulatedtherefrom, said intermediate section including a fourth tube providing afourth outer conductor interconnecting and electrically connected toboth said first tube and said third tube and having the longitudinalaxis thereof disposed at an angle with respect to said third tube, afourth inner conductor disposed within said fourth tube and insulatedtherefrom and having one end thereof connected to said first innerconductor and having the other end thereof connected to said third innerconductor and having the longitudinal axis thereof disposed at an anglewith respect to said third inner conductor, the end of said third tubedisposed toward said generator being outwardly flared to facilitateinsertion thereinto of the adjacent end of said second tube, and a choketube surrounding said third tube and having one end thereof disposedadjacent to the flared end of said third tube and spaced away therefromand having the other end thereof electrically connected to said thirdtube, the distance between the electrical connection of said choke tubeto said third tube and the end of said choke tube adjacent to the flaredend of said third conductor being equivalent to one quarter wavelengthof the frequency of operation of said generator to provide a highimpedance at the flared end of said third conductor to the third andhigher harmonics of the frequency of operation of said generator therebyhighly to attenuate radiation of the third and higher harmonics of thefrequency of operation of said generator from the joint between secondand third tubes.

14. An electronic oven comprising a box-like metal liner defining acooking cavity, a generator for supplying ultra-high frequencyelectromagnetic energy of a pre determined frequency, said generatorhaving an annular outer output terminal and an inner output terminalboth at high potentials with respect to ground potential, and

a coaxial transmission line interconnecting said generator and saidcooking cavity and including a top section coupled to said cookingcavity and a bottom section coupled to said generator and anintermediate section interconnecting said top and bottom sections, saidtop section including a first tube connected to said liner and extendingoutwardly therefrom to provide a first outer conductor and a first innerconductor disposed Within said first tube and insulated therefrom andhaving the outer end thereof coupled to said cooking cavity, said bottomsection including a second tube connected to said annular outer outputterminal and providing a second outer conductor and a second innerconductor connected to said inner output terminal and disposed withinsaid second tube and insulated therefrom, a third tube telescopicallyoverlapping said second tube and insulated therefrom and extendingoutwardly therefrom to provide a third outer conductor, a third innerconductor disposed within said third tube and insulated therefrom andtelescopically overlapping said second inner conductor and insulatedtherefrom, said intermediate section including a fourth tube providing afourth outer conductor interconnecting and electrically connected toboth said first tube and said third tube and having the longitudinalaxis thereof disposed at an angle with respect to said third tube, and afourth inner conductor disposed within said fourth tube and insulatedtherefrom and having one end thereof connected to said first innerconductor and having the other end thereof connected to said third innerconductor and having the longitudinal axis thereof disposed at an anglewith respect to said third inner conductor, the end of said third tubedisposed toward said generator being outwardly flared to facilitateinsertion thereinto of the adjacent end of said second tube, said secondtube having a plurality of spaced apart lands on the surface thereofdisposed toward said third tube, the Width of said lands and the spacestherebetween being such that in cooperation with said third tube thereis provided a choke filter for the third and higher harmonics of thefrequency of operation of said generator further to attenuate radiationof the third and higher harmonics of the frequency of operation of saidgenerator from the joint between said second and third tubes.

15. The electronic oven set forth in claim 14, wherein said lands andthe spaces therebetween are arranged so that the distances between anytwo points of impedance discontinuity on adjacent lands are multiples ofthe wavelength or one-half the wavelength of the third and fourth andfifth and sixth and seventh and eighth harmonics of the predeterminedfrequency of operation of said generator.

16. The electronic oven set forth in claim 14, wherein two of said landsare provided having a space therebetween and on each side thereof, thewidth of each of said lands being equivalent in length to about 0.039times the wavelength of the predetermined frequency of operation of saidgenerator, and the width of the space between said lands beingequivalent in length to about 0.049 times the wavelength of thepredetermined frequency of operation of said generator.

17. An electronic oven comprising a box-like metal liner defining acooking cavity, a generator for supplying ultra-high frequencyelectromagnetic energy of a predetermined frequency, said generatorhaving an annular outer output terminal and an inner output terminalboth at high potentials with respect to ground potential, and a coaxialtransmission line interconnecting said generator and said cooking cavityand including a top section coupled to said cooking cavity and a bottomsection coupled to said generator and in intermediate sectioninterconnecting said top and bottom sections, said top section includinga first tube connected to said liner and extending outwardly therefromto provide a first outer conductor and a first inner conductor disposedWithin said first tube and insulated therefrom and having the outer end17 thereof coupled to said cooking cavity, said bottom section includinga second tube connected to said annular outer output terminal andproviding a second outer conductor and a secOnd inner conductorconnected to said inner output terminal and disposed within said secondtube and insulated therefrom, a third tube telescopically overlappingsaid second tube and insulated therefrom and extending outwardlytherefrom to provide a third outer conductor, a third, inner conductordisposed within said third tube and insulated therefrom andtelescopically overlapping said second inner conductor and insulatedtherefrom, said intermediate section including a fourth tube providing afourth outer conductor interconnecting and electrically connected toboth said first tube and said third tube and having the longitudinalaxis thereof disposed at an angle with respect to said third tube, afourth inner conductor disposed within said fourth tube and insulatedtherefrom and having one end thereof connected to said first innerconductor and having the other end thereof connected to said third innerconductor and having the longitudinal axis thereof disposed at an anglewith respect to said third inner conductor, and a tuning slug extendinglongitudinally outwardly from said third inner conductor at the juncturethereof with said fourth inner conductor, said tuning slug providing acapacitance that cooperates with the distributed capacitance andinductance in said transmission line to provide a low pass filter forfrequencies below the third harmonic of the frequency of operation ofsaid generator and multiples of said third harmonic, whereby in thetransmission of the electromagnetic Wave energy between said third andfourth inner conductors there is effected high attenuation of the thirdharmonic of the frequency of operation of said generator and multiplesof said third harmonic.

18. The electronic oven set forth in claim 17, wherein said second tubeand said third tube telescopically overlap a distance equivalent to aquarter wavelength of the predetermined frequency of operation of saidgenerator, and said second inner conductor and said third innerconductor telescopically overlap a distance equivalent to a quarterwavelength of the predetermined frequency of operation of saidgenerator, thereby to provide high attenuation of the second harmonicand multiples thereof of the predetermined frequency of operation ofsaid generator in the transmission of electromagnetic wave energy alongsaid transmission line.

References Cited UNITED STATES PATENTS 3,172,987 3/1965 Fitzmayer 21910.55 3,321,605 5/1969 Reker 219-1055 3,388,231 6/1968 Staats 219 -1o.55

RICHARD M. WOOD, Primary Examiner,

L. H. BENDER, Assistant Examiner.

